The dehydration and dehydroxylation of alumina gels prepared from the hydrolysis of tri-sec-butoxyaluminium (ASB) at both 25 and 90 degreesC, unmodified and modified with short chain aliphatic acids has been studied using a combination of differential thermal analysis (DTA) and thermogravimetry. The alumina phase in the gel was found to be dependent upon the temperature of hydrolysis and the modifying acid. The gels formed at 25 degreesC from unmodified ASB contained alumina as gibbsite and at 90 degreesC from the unmodified ASB a mixture of gibbsite and boehmite, whereas those gels formed from ASB at 25 degreesC and modified with short chain aliphatic acids were amorphous. In DTA of the modified gels, the exotherms from the combustion of the aliphatic acid groups masked the endotherms of dehydration and dehydroxylation of the gels. Neither DTA nor thermogravimetric analysis (TGA) could determine the dehydroxylation temperature range. The application of differential thermogravimetric analysis (DTGA) enabled these temperatures to be obtained. DTGA of the gels formed from the acid modified ASB showed complex 'endograms'. Infrared spectroscopy was used to determine the molecular structures of the acetic acid modified gels.